Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review presents an overview of recent progress in anti-resonant hollow-core fibers for sensing applications. Both regular and irregular-shaped fibers and their performance in various sensing scenarios are summarized. Finally, the challenges and possible solutions are briefly presented with some perspectives toward the future development of anti-resonant hollow-core fibers for advanced sensing.

The self-assembly of optical solitons propagating in nonlinear dissipative systems spreads the concept of soliton molecules. Assisted with the real-time spectral interferometry, plentiful internal dynamics has been probed within the multi-pulse patterns, emphasizing the striking analogies with the matter molecules. Therefrom, these particle-like behaviors would yield more intriguing landscapes toward the extended degrees of freedom considering increased constituents. Here, we transfer the concept of “isomer” to the experimental investigation on the unexplored isomeric dynamics of soliton molecules in parallel. Particularly, two isomers for soliton triplets and four isomers for soliton quadruplets are captured under different self-assembled forms, within each of which the binding separations and relative phases of the constituents are governed by mutual soliton interactions. With the diverse separation-phase evolving trajectories mapped in the interaction plane, detailed insights into the temporal distribution and the transient dynamics are displayed with respect to a panorama of the isomeric dynamics. The perspective of optical isomers shed new light on the analogy with matter molecules, and the underlying isomeric dynamics may stimulate the artificial manipulation of various soliton molecules for ultrafast applications.

A novel balloon-type photoacoustic cell (BTPAC) is proposed to facilitate the detection limitations of acetylene (C₂H₂) gas achieving ppb level. Here, an ellipsoidal photoacoustic cavity is employed as the platform for gas-light interaction. By strategically directing the excitation source towards the focal point of the ellipsoidal cavity, ensuring its trajectory traverses the focal point upon each reflection from the interior walls. This path increases the gas absorption path length and improves the efficiency of the laser-gas interaction process. Experimental results show that the quality factor of the BTPAC is 224.24 under normal temperature and pressure conditions. Notably, the system maintains a high signal-to-noise ratio even at a C₂H₂ concentration of 0.1 ppm. Moreover, the minimum detection limit (MDL) and normalized noise equivalent absorption can be calculated to be 3.86 ppb, 6.94·10⁻¹⁰ cm⁻¹·W·Hz^−1/2, respectively. Finally, the MDL of the sensor reaches to 0.71 ppb when the integration time reaches 100 s.

An ultrasensitive optical fiber sensor based on the Fabry-Perot interferometer (FPI) and the optical Vernier effect formed by an anti-resonant Bragg hollow core fiber (BHCF) and a single mode fiber (SMF) cascade structure is proposed for measuring ultra-high temperatures. This compact all-fiber structure can be easily fabricated by splicing one end of the BHCF with the SMF used as a light guide, and the other end with the SMF used as a sensor and cut to a specific length. The Vernier effect is formed by the reflection of three beams of light from the SMF-BHCF fusion interface and the two air/glass interfaces at both ends of the sensing SMF. When the length of the two optical fibers meets certain conditions, high-order harmonics Vernier effect can be formed, and the temperature sensitivity can be improved. For the harmonic order i = 1, the temperature sensitivity of the proposed fiber optic sensor is 0.2977 nm/°C in the temperature range of 795-830°C, and the magnification M factor is about 27.59. The fabrication of the optic fiber sensor is straightforward, reproducible, stable, small size, and low-cost, which exhibits potential applications in various industrial processes.

This paper proposes a novel microcone-curved resonant photoacoustic cell (MCR-PAC) for highly sensitive trace gas detection. The MCR-PAC features with microcone-curved resonant region and cylindrical buffer chamber, which dominates the photoacoustic signal amplification. By introducing the hyperbolic eccentricity as a new optimization dimension, the quality factor of the MCR-PAC is remarkably strengthened to enhance the acoustic pressure amplitude. At an eccentricity value of 5, the volume of the photoacoustic resonant cavity is approximately 0.23 cm³. Targeting trace acetylene, the system achieves a minimum detection limit of 1.41 ppb with an integration time of 290 s, corresponding normalized noise equivalent absorption coefficient is 1.88×10⁻⁹ W·cm⁻¹·Hz^−1/2. Compared to the traditional T-type PAC, the overall performance of MCR-PAC has been enhanced nearly fourfold. With its compact millimeter-scale dimensions and high sensitivity, the MCR-PAC demonstrates extensive potential for application in environmental monitoring and breath diagnostics.